An acoustic transducer is an electronic device used to emit and receive sound waves. Ultrasonic transducers are acoustic transducers that operate at frequencies above 20 KHz, and more typically, in the 1-20 MHz range. Ultrasonic transducers are used in medical imaging, non-destructive evaluation, and other applications. The most common forms of ultrasonic transducers are piezoelectric transducers. In U.S. Pat. No. 6,271,620 entitled, “Acoustic Transducer and Method of Making the Same,” issued Aug. 7, 2001, Ladabaum describes microfabricated acoustic transducers capable of competitive performance compared to piezoelectric transducers.
The basic transduction element of the microfabricated ultrasonic transducer (MUT) described by this prior art is a vibrating capacitor. A substrate contains a lower electrode, a thin diaphragm is suspended over the substrate, and a metallization layer serves as an upper electrode. If a DC bias is applied across the lower and upper electrodes, an acoustic wave impinging on the diaphragm will set it in motion, and the variation of electrode separation caused by such motion results in an electrical signal. Conversely, if an AC signal is applied across the biased electrodes, the AC forcing function will set the diaphragm in motion, and this motion emits an acoustic wave in the medium of interest.
FIG. 1 illustrates the naming conventions of orientation and direction used in ultrasound engineering. As shown in FIG. 1, the transducer array 100 is typically made up of multiple transducer elements 110. In the case of capacitive microfabricated transducers, each of the transducer elements 110 includes a plurality of individual transducer cells. The transducer elements 110 are oriented such that their lengths are along the elevation axis, and their widths are along the azimuth axis. The transducer elements 110 are adjacent to one another along the azimuth axis.
Currently, the most common forms of ultrasound imaging systems generate images by electronic scanning in either linear format or sector format. FIG. 2 illustrates the linear 210 and sector 220 image formats generated by a typical ultrasound system. In linear format scanning, time delays between transducer elements are used to focus the ultrasound beam in the image plane. In sector format scanning, time delays between transducer elements are used both to focus the ultrasound beam and to steer it. Typically, the sector scan format 220 is used to image a relatively large, deep portion of the anatomy from a small acoustic window (e.g., imaging the heart); whereas the linear scan format 210 is used for optimum image quality near the face of the transducer (e.g., imaging the carotid).
An ultrasound imaging system, whether in linear or sector format, forms an image by combining the results of many transmit-receive sequences. Each transmit-receive event is commonly referred to as a “beam” or a “vector” because appropriate delays are applied between the transmit waveform of each element such that the transmit energy is directed in a specific direction towards a specific region of the image being formed, and delays are dynamically applied to the received waveforms of the elements such that a line of the image can be formed along this direction. FIG. 3 illustrates, for a convex linear probe 300, made of elements 310, a set of vectors 320 used to form the image. A complete image frame is formed from approximately 100 to 300 hundred such vectors, and ultrasound systems generate approximately 30 frames per second.
Harmonic imaging is an important modality in diagnostic ultrasound. Harmonic imaging results when the subject of interest is interrogated with ultrasonic waveforms centered around frequency f, and then the return, or echo, signal is detected around a harmonic frequency of f, for example, 2f. Human tissue generates harmonics, as do contrast agents. Harmonics need not be limited to 2f, they can be 3f or higher, or sub-harmonics. It is very important for harmonic imaging that the transmitted ultrasound be free of harmonics, or that these transmitted harmonics be subtracted out in subsequent received waveform signal processing.
Capacitive transducers can transmit harmonics because the force on the diaphragm is proportional to the square of the applied voltage excitation waveform. Further non-linearity stems from the fact that the force on the diaphragm is also dependent, in a quadratic manner, on the position of the diaphragm relative to its resting state. Because broadband transducer designs have diaphragms that respond to such non-linear forcing functions in a meaningful manner, they transmit harmonics. Harmonic transmission from the transducer is undesirable in tissue harmonic imaging and contrast agent imaging because these imaging modalities are based on forming images with harmonics generated by the tissue or the contrast agent, not by the harmonics transmitted by a sub-optimal transmitter.
The use of pre-distorted input signals in electronic systems so as to reduce the harmonic content of an output signal is a technique that has been used in electronics for a long time and is well known in the art. For example, Holbrook et al., in U.S. Pat. No. 2,999,986 issued in 1961, teach a pre-distortion technique to reduce harmonics generated by a non-linear vacuum tube. Savord et al. received U.S. Pat. No. 6,292,435 for the application of pre-distorted signals to capacitive microfabricated ultrasonic transducers (cMUT). Fraser received U.S. Pat. No. 6,443,901 also for the application of pre-distorted signals to cMUTs. Hossack, in U.S. Pat. No. 6,461,299 teaches different pre-distortion methods to those taught in Savord et al. and Fraser. Savord et al., Fraser, and Hossack exclusively teach pre-distortion approaches to remove harmonics from the transmit signal. Pre-distortion approaches place design challenges on a system's transmitter. At best, they require a sophisticated and relatively expensive transmitter. At worst, the approach requires an entirely new ultrasound system to operate cMUTs in harmonic imaging mode.
In U.S. Pat. No. 5,233,993, Kawano teaches a method whereby an ultrasound system forms an image based on the combination of two echoes from two transmit signals in the same scanning direction. In U.S. Pat. No. 5,632,277 Chapman et al. teach a method of generating an ultrasound image that enhances regions of non-linear scattering media by using two transmit signals 180 degrees apart in phase. In such an approach, the received echoes from linear media will be opposites of each other and cancel if added, but if a region is non-linear, there will be no significant difference in the received echoes of the harmonic energy. Further, Hwang et al., in U.S. Pat. Nos. 5,706,819 and 5,951,478, teach specifics of such an approach for imaging with contrast agents. Averkiou et al., in U.S. Pat. No. 6,186,950, introduce improvements to such pulse inversion harmonic imaging by using more than two temporally spaced transmit pulses per pulse-echo sequence. U.S. Pat. Nos. 5,902,243 to Holley et al. and 5,961,463 to Rhyne et al. teach specifics of useful transmit waveforms. Common to all such prior art is that the method taught for producing suitable transmit waveforms uses the signal generator of the ultrasound system to distort, encode, or sequentially phase invert the transmit waveforms of each element, where the only relationship of the waveforms of adjacent elements is that governed by the appropriate delays in the azimuth direction.
In the '638 provisional application to Panda et al., methods of combining bias polarity patterns and multiple firings are taught that enable the cancellation of transducer-emitted harmonics. Panda et al. teach that a tight spatial distribution of alternating bias polarity across a cMUT element's aperture results in a transducer whose fundamental content is effectively canceled, but whose even harmonic content is the same as that of a cMUT with the same polarity bias across its aperture. Panda et al. present a mode of operating a cMUT in such a way that only its even harmonic content effectively radiates. When used in a method of multiple transmit firings and received signal combinations, this harmonic-only mode of operation can be used to remove the effects of cMUT generated harmonics. The disadvantage of the '638 provisional application is that it requires a cMUT specifically fabricated with electrodes and/or connections such that bias polarity in the elevation direction can be varied, and a system capable of supplying the control for such bias electrodes and/or connections.
It is therefore desirable to provide a method of operating a cMUT in harmonic imaging without necessitating pre-distorted waveforms or elevation bias control.
The present inventors have realized that by simply inverting the transmit waveform to adjacently spaced azimuth elements, and combining at least two additional firings without adjacent inversion for each transmit vector, the second harmonic generation (and other even harmonics) of the cMUT can be canceled, and thus harmonic imaging with cMUTs can achieve improved performance.